1. Field Of The Invention
The present invention relates to an improved acetabular prosthesis for use in a total hip joint prosthesis which uses materials that conform and adapt to the shape of the natural acetabulum of the patient.
2. Description Of The Prior Art
A total hip prosthesis commonly includes two components: a femoral-head prosthesis and an acetabular prosthesis.
A known femoral-head prosthesis includes a ball-type head which has the shape of a hemisphere. A stem extends from the head and is adapted for insertion into a femoral medullary canal. The stem is preferably coated with a stabilizing material, either an acrylic cement or a low modulus tissue ingrowth material, such as is disclosed in U.S. Pat. No. 3,992,725 dated Nov. 23, 1976.
One known type of acetabular prosthesis includes a hemispherical cup which is fixed within the natural acetabulum with an acrylic cement and through anchoring holes in the pelvis. Screws or the like extend from the convex side of the cup and engage the holes in the pelvis.
But, when the cement non-uniformly transfers load stresses, there can result a loss of bone leading to a gradual degradation of the useful life of the implanted prosthesis.
An acetabular prosthesis is also known which has a metallic hemispherical cup which is not mechanically secured to the pelvis. The cup has an internal concave surface which is lined with a biocompatible and wear resistant polymer such as ultra high molecular weight polyethylene. Articulation in this implant takes place on one hand between the head of the femoral-head prosthesis and the lining of the cup, and on the other hand between the polished convex surface of the cup and the natural acetabulum.
While such an unfixed acetabular prosthesis does not require holes in the pelvis, it may develop serious problems due to the fact that the exterior of the cup is hemispherical and the natural acetabulum in which the cup articulates is not a true portion of a sphere because it may have natural or disease-caused irregularities, which can cause loads transmitted through the hip joint to become localized and abnormal, thereby damaging the natural acetabulum. Such localized load is also understood to be a cause of pain.
This problem is described in U.S. Pat. No. 4,318,191, wherein a unitary hip joint prosthesis is described having a compliant metal head which is intended to conform and adjust to the shape of the natural acetabulum and thereby to avoid abnormal stress loadings. This metal head leaves much to be desired as to its ability to adjust to irregularities in the acetabulum geometry and to protect the acetabulum against abnormal dynamic loads which are normally transmitted by the hip joint.
Another prosthetic device is described in U.S. Pat. No. 4,164,794 dated Aug. 21, 1979 and has a coating particularly designed for use where long-term bone fixation is desired by tissue ingrowth into and through the coating with subsequent remodelling to bone. The porosity and other characteristics of the coating are designed to promote tissue ingrowth so that stresses are transferred to bone spicules within the pores. A relatively high modulus of elasticity is provided in U.S. Pat. No. 4,164,794 that is conductive to bone formation without micromotion occurring with loading of the prosthesis.